<!DOCTYPE html>
<!--
 | The rules for non-replaced block-level elements are applied to determine the
 | margins.
 |   https://www.w3.org/TR/CSS21/visudet.html#block-replaced-width
 |
 | The following constraints must hold among the used values of the other
 | properties:
 |       margin-left + border-left-width + padding-left
 |     + width
 |     + padding-right + border-right-width + margin-right
 |     = width of containing block
 |
 | If there is exactly one value specified as "auto", its used value follows
 | from the equality.
 |   https://www.w3.org/TR/CSS21/visudet.html#blockwidth
 -->
<html>
<head>
  <style>
    .containing-block {
      background-color: #2196f3;
      padding: 10px 0;
      margin-bottom: 10px;
      width: 500px;
    }
    .block {
      /* TODO: Check that border width is taken into consideration by
             constraint formula. */
      /* TODO: Investigate behaviour when using non-black background.
             When a color other than black is used here, a spurious black box is
             rendered inside the video element that is not visible on Chrome. */
      background-color: #000;
      display: block;
      height: 5px;
      padding: 0 100px;
    }
    .auto-margin-left {
      margin-left: auto;
      margin-right: 50px;
      width: 100px;
    }
    .auto-margin-right {
      margin-left: 50px;
      margin-right: auto;
      width: 100px;
    }
    .auto-width {
      margin-left: 50px;
      margin-right: 50px;
      width: auto;
    }
  </style>
</head>
<body>
  <div class="containing-block">
    <video class="auto-margin-left block"></div>
  </div>
  <div class="containing-block">
    <video class="black auto-margin-right block"></div>
  </div>
  <div class="containing-block">
    <video class="auto-width block"></div>
  </div>
</body>
</html>